The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Breaking the usual protocol of Australian politicians not commenting on overseas election campaigns, Mr Pyne said the violence and farcical scenes at Trump's rallies made for uncomfortable viewing.

"Democracy should be robust but it certainly shouldn't be violent. And I think the Donald Trump phenomenon is a real problem for the United States, making their democracy look kind of weird," Mr Pyne told Channel Seven this morning.

"It's terrifying, and I think for the Republican party, if they choose Donald Trump, will find themselves in the wilderness for a very long time."

Conversely, conservative Liberal senator Cory Bernardi said the rise of Trump shone a light on "global disenchantment with mainstream politics".

"I have been warning about this for some time. Unless the major parties respond to the concerns of mainstream people, the public will look elsewhere and a more formidable force will emerge," Mr Bernardi said.

While he is the frontrunner for the Republican presidential nomination, political observers believe Trump may fall short of the 1237 delegates needed for an outright win.